A gel consists of a liquid phase that is held together by a continuous solid network. A unique and desirable property of a gel is that molecules in the liquid phase are free to diffuse throughout the bulk of the gel, but the liquid itself is geometrically constrained. As a result, gels have been used in applications where high molecule mobility is desirable, but it is advantageous to confine the liquid without having to surround it completely with walls, such as in chemical separation materials or battery electrolytes.
The continuous solid network that constrains the gels can be made using a number of classes of materials, including: agglomerated particles, fibers, or plates; polymers joined by small crystalline regions; polymers linked by covalent bonds; and polymers linked by non-covalent bonds, such as hydrogen bonds or coordination compounds.
Typically, the solid network occupies less than 10% of the volume of the gel. At higher solids loading, liquid can no longer diffuse independently from the solid, and diffusivity can decrease dramatically. This shortcoming is particularly acute for systems gelled by polymers, where the mixing between the liquid and the solid occurs on the molecular level. In order to obtain good diffusivity, the gels tend to be lightly loaded with solids and mechanically quite weak.
Gels made using inorganic materials as the solid component can generally have improved mechanical properties relative to those using polymers as the gelling agent. This can arise in part from the geometry of the dispersed solid: a rigid inorganic fiber or rod, spanning microns through a liquid, can bear a larger load than individual strands of polymer. Using sol-gel chemistry, inorganic materials can also be made that have regions of greater density (which exclude liquid), adjacent to regions of lower density (which can be predominately liquid). Continuous regions of greater density can bear a mechanical load, while regions of lesser density can allow for the transit of molecules in the liquid phase.
Sol-gel chemistry, however, can be relatively difficult to optimize and control, as the result obtained by one experiment performed at a specific temperature, solvent, salt concentration, aging protocol, etc. will not yield the same result when any of the parameters are changed. Parameters such as Young's modulus, fracture toughness, etc. can change dramatically with even changes in the thickness of the film. Furthermore, it can be difficult to guarantee a uniformity of pore size, a parameter which is critical for many applications. As a result, it would be preferred to find a gel system that has a wider process window and gives more uniform, predictable results.
Nanoparticles have been used to gel liquid systems such as aqueous sulfuric acid battery electrolyte. Typically silica particles have been used, as in U.S. Pat. No. 3,271,199, but other nanoparticles have also been applied, including organic ones as in U.S. Pat. No. 3,930,881. Generally, agglomerated particles of any type can gel a liquid, but the total particle loading of the gel is limited by the ability to homogenously mix the two components. Because the stiffness of the gel increases as the particle fraction increases, mixing often becomes difficult above a 10% particle loading, with this critical loading varying from system to system. As a result, this method has difficulty producing gelled system with sufficient stiffness to act as battery separators, as described in Li et al. [Y. Li, J. A. Yerian, S. A. Kahn, P. S. Fedkiw, “Crosslinkable Fumed Silica-Based Nanocomposite Electrolytes for Rechargeable Lithium Batteries”, Journal of Power Sources, 161 (2006) p. 1288-1296].
In all cases, what is desired is a two phase system, where a solid phase serves as a porous mechanical support, and a second, liquid phase can flow freely through the pores. Such a system would be maximally useful if the two phases are separated on the nano-scale, as this would enable the material to appear homogenous on applied length scales.
A liquid system that has been gelled by a compatible polymer is essentially a single phase, and the resulting material exhibits neither the high diffusivity of the parent liquid nor the mechanical properties of the parent polymer. However, if the liquid and polymer are not compatible, the two materials will phase separate. Thus, for the vast majority of polymers and liquids, it is not possible to come up with a system with the desired attributes of good materials properties and good liquid diffusivity.
One example of a system with some success in this endeavor was demonstrated by Gin et al. [Gin, D. L.; Gu, W.; Pindzola, B. A.; Zhou, W.-J. “Polymerized Lyotropic Liquid Crystal Assemblies for Materials Applications,” Acc. Chem. Res. 2001, 34, 973-980], where nanochannels of water are supported by a polymerized liquid crystal scaffolding. In this system, the liquid crystalline order is needed to keep the solid phase intermingled with the liquid phase on the nanoscale. The drawback of this approach is that, so far, these materials must be processed in carefully controlled conditions to preserve the liquid crystalline order, thus limiting its utility as a technology.
Two fields that would benefit most from a mechanically stable gelled liquid are: gelled electrolytes, for instance for batteries, supercapacitors, or electrochemical sensors; and constrained liquid membranes, for gas separations.